Electrode for plating hollow articles



April 24, 1956 R. H. HILL ET AL 2,743,229

ELECTRODE FOR PLATING HOLLOW ARTICLES Filed March 5, 1952 lw \v Hmhefi: H Hi11 Alfred P. Knu

United States Patent ELECTRODE FOR PLATING HOLLOW ARTICLES Application March 3, 1952, Serial No. 274,558

6 Claims. (Cl. 204-290) This invention relates to electrodes and more particularly to electrodes for use in plating the interior surfaces of generally tubular articles.

In the electroplating of the interior surfaces of elongated tubular articles with hard coatings such as chr0- mium, the hollow article to be coated is generally disposed in an upright position and serves as the cathode. An electrode having a power supply cable attached to the upper end thereof is centrally located within the article and extends through the entire length thereof to serve as the anode. The plating solution or electrolyte fills the annular space between the anode and the walls of the article.

However, with this type of cathode and anode construction, diifieulty is invariably experienced in obtaining a deposit of substantially uniform thickness throughout the length of the surface to be plated. For one thing, the passage of electric current through the electrolyte from the anode to cathode generates gaseous products which appreciablyimpede the ionization of the electrolyte. Although these gases do rise upwardly in the electrolyte with sufiicient rapidity to produce acertain amount of pumping action, the consequent circulation'of the electrolyte is not suflicient to prevent a progressive upward increase in gas density as the quantities generated in the lower regions thereof rise and merge with those generated in the upper regions. Thus, since the current-carrying capacity of the electrolyte is reduced in proportion to the quantity of gases present at any particular point therein, a heavier chromium coating is produced at the lower portions of the article tapering to a thinner coating at the upper portions thereof. This difference in coating thickness cannot be eliminated merely by increasing the supply of current since the same relative increase will be delivered to the lower regions of the electrolyte. In fact, such increase in current has a tendency to burn the electrode with the consequent deposition of an imperfeet coating on the article. In addition, the heat produced in the electrolyte by an increase in current actually adds to the quantities of objectionable gases generated therein.

Another difficulty in plating long tubular articles lies in the voltage drops which occur in both the anode and cathode as a result of the linear resistances thereof. These voltage losses produce a lower ampere per square inch current value at the bottom regions of the electrolyte than at the top regions thereof and, therefore, theoretically result in lessdeposit of chromium at the bottom of the article to be plated than at the top thereof. Since this difference in deposit is the direct opposite of that produced by theaf'orementioned gas accumulation, it can readily be seen that the thickness of the total deposit falls short of the optimum which can be expected in the plating .of relatively short articles where these undesirable conditions do not exist to any appreciable extent.

While small differences in coating thickness along the length of a tubular article may not be objectionable in many instances, the ballistic requirements of gun barrels Patented Apr. 24, 1956 ranging between 10 and 25 feet in length and between 2 and 5 inches in diameter necessitates a coating which will not vary more than .001 in thickness throughout the entire length thereof. In fact, any greater taper or other variation in the uniformity of plating thickness is so objectionable that present day methods of finishing gun barrels invariably include the step of honing the bore surface thereof after plating. Needless to say, such procedure is both expensive and time-consuming due to the special equipment required.

Previous attempts at preventing a tapered chromium plate in gun barrels of the aforesaid length have involved mechanical means for increasing the circulation of the electrolyte in order to disperse the gases generated therein with sufiicient rapidity to prevent any appreciable accumulation thereof. However, those attempts which did meet with some measure of success are objectionable due to the complicated and time-consuming procedures .involved and to the elaborate and expensive apparatus invariably required. In still another attempt, the gases have been exhausted by providing a hollow anode having suitable openings along the length thereof. This solution, however, is impractical in the chromium plating of 1 long gun barrels where the electrode must possess sufficient rigidity to withstand the relatively large; magnetic torque exerted thereon during the initial application of the electric current. In addition, the electrode must be strong enough to undergo prolonged use without distortion. Any flexing or bowing of the electrode will obviously disturb the linear concentricity thereof and consequently produce an uneven coating on the bore surfaces of the gun' barrel. 7

In order to overcome difficulties such as those referred to hereinabove, a traveling electrode has been utilized; In this expedient, an electrode is carried at the lower end of an insulated vertically extending cable which is in turn wound around a rotatable drum located above the vertically-positioned gun barrel. During the plating operation, the drum is slowly rotated so as to wind the cable and move the electrode lengthwise through the barrel and the electrolyte therein. This equipment, however, is unduly complicated and therefore too expensive to operate, especially where the economies of mass production are being sought. An even more serious objection is the length of time required to satisfactorily complete the required plating thickness. In many instances, as much as .ten or twelve hours are required to coat a single gun barrel.

Accordingly, it is an object of this invention to provide an improved electrode adaptedfor use in the plating of the interior surfaces of hollow articles.

Another object of this invention is to provide an improved electrode adapted to be used in forming coatings of substantially uniform thickness along the walls of relatively long hollow articles.

, Still another object of this invention lies in the pro vision of an improved electrode adapted to be-used in simultaneously plating chromium along thefull length of the interior surfaces of relatively long hollow articles.

It is a further object of this invention to provide an improved electrode which will be relatively inexpensive to manufacture and which will possess a high degree of resistance to distortion during use in the chromium plating of the interior surfaces of relatively long hollow articles.

It is a specific object of this invention to provide an electrode which is particularly useful in electroplating chromium on the bore surfaces of relatively long gun barrels.

The specific nature of the invention as well as other objects and advantages thereof will clearly appear from a description of a preferred embodiment as shown in the accompanying drawings in which:

Fig. l is a longitudinal view of theelectrode of the present invention, shown partly in cross-section;

Fig. 2 is a top view of the electrode of Fig. 1;

Fig. 3 is a cross-sectional view taken along the line 33 of Fig. l; and

Fig. 4 is a cross-sectional view of an alternate form of electrode constructed in accordance with the teachings of the present invention.

As shown in Figs. 1-3, the preferred electrode of this invention essentially comprises an upper copper conducting core 5 and a lower steel stiffening core 6, both onclosed within a protective lead covering 7. Core 5 is of cylindrical construction and is preferably fabricated from solid hard-drawn copper. Core 6 is also of solid cylindrical construction but the diameter thereof is relatively smaller than core 5. Core 6 is preferably fabricated from stainless steel and is surrounded by an adherent an nular copper conducting layer 8 of such thickness that the total diameter of steel core 6 and copper layer 8 does not exceed that of copper core 5. Layer 8 may extend to the bottom end of core 6 or, as shown in Fig. 1, may stop short thereof. Copper layer 8 is preferably formed by any of the well-known spraying processes although, as an alternate method of application, copper layer 8 may also be bonded to steel core 6 by brazing. However, this latter method is less desirable than the spraying process since there is a tendency for the brazing heat to warp or otherwise twist steel core 6 out of shape. In addition, there also exists the possibility of producing occasional voids in copper layer 8 which must be carefully removed in order to maintain the optimum current flow therethrough so necessary for achieving the best results in plating.

Copper core 5 and steel core 6 are joined together by any suitable connecting means which, as shown in Fig. 1', may comprise a threaded stud portion 9 projecting upwardly from steel core 6 and arranged to mate in a correspondingly threaded recess 10 provided in the bottom of copper core 5. V For greater rigidity, cores 5 and 6 may be brazed at adjacent ends as shown at 11. This brazing does not cause any objectionable warping or deformation of steel core 6 since the heat thereof is applied over a relatively small area. If a superior electrical connection is desired, the joining of cores 5 and 6 may be accomplished by silver soldering.

Lead covering 7 is preferably applied by any suitable extrusion process which tends to produce a homogeneous coating tightly bonded to both the underlying copper and steel surfaces. A tight bond avoids the possibility of a loss in current or any interference with the flow of current between the copper and the lead. The thickness of those portions of lead covering 7 which surround copper layer 8 and steel core 6 is such as to bring the external peripheries of these lead portions into coincidence with that of the portion surrounding copper core -5 thereby producing a cylinder having the same external diameter throughout. Lead covering 7 is applied to the entire exterior periphery of the electrode so as to come into contact with the chromic acid or other ingredients of the plating bath and thus protect the interior copper and steel portions from being attacked by the bath liquid. The upper end of core 5 projects upwardly beyond lead covering'7 and terminates in an integral rectangular portion 12 having a suitable aperture 13 therein for facilitating the connection thereto of an electric fixture or conducting cable (not shown).

The lengths of copper core 5, steel core 6, and copper layer 8 will generally depend upon the length of the hollow article whose interior is to be plated. For example, where the electrode is to be utilized for plating a gun barrel having a length of about fifteen feet, copper core 5 may have a length of about seven and one half feet while copper layer 8 may have a length of about nine and one half feet. The steel portion may project any suitable distance below copper layer 8, if such projection is desired, depending upon the length of the article to be plated. The over-all greater length of the electrode allows projection of either end thereof beyond the breech and muzzle ends of a vertically suspended gun barrel for connection with suitable retaining and conducting fixtures. Where the gun barrel has an internal diameter of about three and one half inches, copper core 5 may be about two inches in diameter and lead covering '7 at that section may be about one eighth of an inch thick, thereby making an over-all dimension of about two and one quarter inches. The steel core 6 at the lower part of the electrode may be about one and a quarter inches in diameter, the copper layer 8 about one eighth of an inch thick, and the outermost lead covering 7 may have a thickness of about three eighths of an inch, thereby giving an electrode which has an external diameter of about two and one quarter inches along the entire length thereof. in such case, the thickness of lead covering 7 about the bottom end of steel core 6 and about the sec tion thereof not covered by copper layer 8 may have a thickness of one half of an inch.

When the electrode is suspended centrally within a gun barrel and the space between the electrode and the walls of the barrel is filled with a suitable electrolyte, as, for example, chromic acid, current supplied to rectangular portion 12 of copper core 5 passes therethrough to steel core 6 and copper layer 8. The relatively large copper cross-section of core 5 provides a greater flow of current to the upper regions of the electrolyte than that supplied to the lower regions by the less-conducting steel core 6 and the smaller cross-section of copper layer 8.

Accordingly, copper core 5 passes a relatively large portion of the total supply of current to steel core 6 where the conductivity thereof is less than that of copper, and hence a lesser current is delivered to the lower regions of the electrolyte than would be the case were core 6 fabricated entirely from copper. Thus, the current supplied by copper core 5 to the upper regions of the electrolyte is sufficient to overcome the resistance thereto introduced by the progressively increasing gas accumulation in such regions and thereby maintains a definite ampere per square inch cathode current density. Although the progressive linear cross-section of copper core 5 offers a corresponding increase in resistance to the passage of electric current, the resulting voltage loss is proportional to the simultaneous progressive decrease in the density of the gases in the electrolyte.

The density of the gases generated in the electrolyte decreases at a faster rate in the area coinciding with steel core 6 than in the upper regions about copper core 5. However, the combined resistance of steel core 6 and copper layer 8 thereon is not great enough to block delivery of sufiicient current to maintain the same rate of chromium deposit as occurs in the upper regions of the electrolyte about copper core 5. The correspondingly greater decrease in the delivery of electric current to the lower regions of the electrolyte about the leadcovered portion of steel core 6 is produced by the increased resistance thereof resulting from the absence of copper layer 8. Hence, the undesirable taper commonly produced in gun barrels by the prior art types of anodes is here virtually eliminated since the electrical resistance of the above-described composite electrode is such as to compensate for the progressively decreasing gas density found in the electrolyte.

In some instances, it may be desirable to utilize an electrode comprising a stiffening steel core 15 extending substantially through its entire length, a layer of conducting material such as copper secured to and extending along the external surface of the steel core, and an external lead covering 16 for protecting the copper and steel from attack by the acids in the electrolyte. An electrode of this type is desirable where the generation of the gases is not great enough to offer appreciable resistance to the flow of electric current through the electrolyte. Such an electrode is also desirable where extremely accurate positioning thereof relative to the articlewalls is required and Where such accurate positioning must be retained throughout the entire length of the electrode.

As shown in Fig. 4, this type of electrode may have a heavier copper layer 17 at the upper portion of core 15 than at the lower portion thereof as indicated by 18. The junction formed by annular layers 17 and 18 is brazed as shown at 19 for greater rigidity and lead covering 16 is superimposed on the exterior thereof from a predetermined point on copper layer 17. A clamp or collar (not shown) may be secured to the upper end of the electrode to supply current thereto. This construction operates similarly to the preferred embodiment previously described and produces a deposit of uniform thickness throughout the length of the article to be plated.

In both electrodes, the dimensions of each portion thereof depend upon the diameter and length of the articles to be plated as well as the type of plating solution employed, the thickness of plate desired, and the maximum available intensity of the electric current. These factors can be analyzed and mathematically computed to determine the varying resistances necessary at the different portions of the electrode. However, in some cases it may be necessary to determine the, electrode dimensions by actual experimentation.

The results achieved in chromium plating gun bores by utilizing the type of electrodes herein described are far superior to those obtained with previous expedients and methods. It has been possible to plate a hard layer of chromium along the entire bore surface of barrels up to twenty-five feet in length with such uniformity of thickness that no subsequent honing whatever is required. Ample current is supplied to both the upper and lower regions of the electrode and thereby to the electrolyte so that adequate compensation may be made for any interference which limits the rate of chromium deposit. Of equal importance is the fact that the period of time required to plate these long gun barrels has been reduced to about three or four hours as compared with the ten or twelve hours required .in the use of a traveling electrode. Moreover, the use of stainless steel for the ferrous .cores of these novel electrodes not only serves to provide the necessary resistance to the flow of electric current therethrough but also provides suflicient stiffening thereof to adequately withstand the torque forces exerted thereon due to the heavy magnetic flux existing in the flow of high current. The rigidity and durability of these electrodes permits their use in the etficient and accurate plating of the entire bore surfaces of well over two hundred of the long gun barrels employed in heavy ordnance materiel. When the continued use of the electrodes reduces the efliciency of plating, it is merely necessary to remove the worn lead covering and apply a new one since the life of. the remaining portions of the electrode is virtually indefinite.

We claim:

1. An anode for plating a uniform layer of metal on the bore surface of a tubular article comprising an upper portion having a copper core, a lower portion having a stiff core of material less conductive than copper, an annular copper portion encircling said stilf core in electrical connection with said copper core, said stiif core projecting beyond the lower end of said annular copper portion, and a protective lead casing about said copper core, said annular copper portion, and said projecting end portion of said stiff core.

2. An anode as defined in claim 1 including threadable means for securing said stifi core to said copper core.

3. An anode as defined in claim 1 wherein said annular copper portion is bonded to the exterior of said stiff core and said lead casing is bonded to the exterior of said copper core, to the exterior of said annular copper portion, and to the exterior of said projecting end portion of said stiff core.

4. An anode as defined in claim 1 wherein an end portion of said copper core projects above said lead casing and is provided with means for facilitating connection to a power supply.

5. An anode as defined in claim 1 wherein said stiff core is of lesser diameter than said copper core, and wherein the thickness of said lead casing about said annular copper portion is greater than that about said copper core but lesser than that about said projecting end portion of said stiff core in order to equalize the external diameter of the anode throughout the full length thereof.

6. An anode for plating the bore surfaces of elongated generally tubular articles such as gun barrels comprising a copper core having a centrally disposed threaded recess in the lower end thereof, a ferrous core having a threaded stud projecting from the upper end thereof and engageable in said threaded recess for longitudinally connecting said copper and ferrous cores, an annular copper layer encircling said ferrous core in electrical connection with said copper core, said ferrous core having a lower end projecting below said annular copper layer, and a protective lead covering about said copper core, said annular copper layer and said projecting end of said ferrous core, said copper core having an upper end projecting above said lead covering and provided with means for facilitating connection to a power supply.

References Cited in the file of this patent UNITED STATES PATENTS 

1. AN ANODE FOR PLATING A UNIFORM LAYER OF METAL ON THE BORE SURFACE OF A TUBULAR ARTICLE COMPRISING AN UPPER PORTION HAVING A COPPER CORE, A LOWER PORTION HAVING A STIFF CORE OF MATERIAL LESS CONDUCTIVE THAN COPPER, AN ANNULAR COPPER PORTION ENCIRCLING SAID STIFF CORE IN ELECTRICAL CONNECTION WITH SAID COPPER CORE, SAID STIFF CORE 